Composite electro-acoustic transducer configuration



1961 D. R. CHURCH ETAL 2,998,535

COMPOSITE ELECTRO-ACOUSTIC TRANSDUCER CONFIGURATION Filed April 29. 1958 INVENTORS. 05527 L. E00

1 E, ao/V410 2. 0/1/201 This invention relates to electro-acoustic transducers, and more particularly to transducers used with isolation means. The primary object of the present invention is to generally improve such transducers.

In the design of electro-acoustic transducers of the magnetostrictive, piezoelectric and ferroceramic types, it is frequently necessary to isolate a vibrating transducer element from the load or, in the reciprocal ease, to isolate a pickup transducer from its vibrating source. Such isolation devices usually consist of coupling slugs having optimized acoustic properties and dimensions which enhance the transmission of the vibratory energy with a minimum of attenuation.

For those applications involving widely different working temperatures at the load and the transducer, it becomes difiicult to fabricate a suitable coupling section having structurally strong interfaces free from air or vapor entrapments. Further, the interfaces must be capable of withstanding the enormous stresses involved in vibrations of high frequency, stresses which invariably involve high particle velocities and accelerations even though the amplitudes of motion are small.

In the particular cases of piezoelectric and ferroceramic transducers, it is not readily possible to cement coupling sections to transducer elements, while soldering techniques are not much more suitable, involving as they do the creation of excessive temperatures. In an attempt to alleviate these difficulties, it is standard practice to cement such piezoelectric or ferroceramic elements to their loads or coupling section by means of epoxy type large temperapresent in the various parts of the of ferrois used to sense the presence or absence of liquid. A cylindrical slug of metal is cemented to the ceramic.

In both of the above cement bond may break differential is set up between the transducer element The general object of the present invention is to overcome the foregoing difficulties, and to do so in a rather dtates Patent simple manner. Our invention eliminates the difference in thermal expansion between the transducer and a cou pling or isolation section, and provides a high degree of thermal isolation, which is valuable when the load is extremely hot and above the safe operating temperature of the heat sensitive transducer material itself. For this purpose we provide a composite transducer assembly in which the vibrating element itself, and the coupling or isolation slug, are of one and the same material.

To accomplish the foregoing general objects, and other more specific objects which will hereinafter appear, our invention resides in the combined transducer and isolation elements, and their relation one to another as are hereinafter more particularly described in the following specification. The specification is accompanied by a drawing in which:

FIG. 1 is a perspective view, drawn to reduced scale, showing a conventional transducer, for contrast with our improved transducer;

FIG. 2 is a perspective view, drawn to larger scale, showing a combined transducer and isolation means embodying features of our invention;

FIG. 3 is a perspective view showing a modified form of our invention in which the entire assembly is made of a single integral body of transducer material;

FIG. 4 shows a further modification of the invention in which the transducer is provided with an integral mounting flange;

FIG. 5 is a perspective view showing the same features applied to a different form of transducer intended for liquid level sensing;

FIG. 6 is a schematic view showing the transducer of FIG. 4 applied to an ultrasonic cleaning apparatus;

FIG. 7 is a schematic showing of the invention applied to a soldering iron; and

FIG. 8 is a schematic view showing the invention applied to a magnetostrictive transducer assembly.

Referring to the drawing, FIG. 1 shows a conventional transducer for purposes of comparison with and explanation of the invention. In FIG. 1 there is a solid block of transducer material 12, to the ends of which are deposited fired silvered electrodes 14 and 16, which are connected at tabs 18 and 20 to electrical leads 22 and The transducer materi 12 is suitably polarized, as one step in the manufacture of the transducer, and thereafter when a source of alternating voltage is applied across the leads 22, 24 a mechanical motion is obtained as indicated by the arrow 26. The alternating voltage may be supplied from a conventional high frequency generator, and the applied frequency preferably corresponds to the natural resonance frequency of the transducer block 12, in this case in the length mode, that is along the longitudinal or vertical axis indicated by the arrow 26. A sinusoidal mechanical motion is obtained at the ends.

For purposes of example, and for future reference and comparison, we may mention that a block of solid barium titanate material, measuring about 2 inches high, resonates in its natural fundamental length mode at a frequency of approximately 40,000 cycles per second.

Even more importantly, damage to the transducer block itself may result from the heat conducted thereto, for these materials are sensitive to excessive heat. For one thing, heat may destroy the polarization of the material.

Referring now to FIG. 2, the assembly there shown has the same physical dimensions as the slab shown in assesses PEG. 1, although the latter is drawn to half the scale of the former. Accordingly, the transducer material 30, 32 is dimensioned for vibration at the same desired frequency. It comprises both a transducer, which is the lower portion 30, and an isolation means or slug, which is the upper portion 32. There are means 34, 36 to electrically energize the transducer portion 30, while the isolation portion 32 is not energized. Preferably it is only the energized portion 30 which is polarized whereas the isolation portion 32 is left unpolarized. In the specific case here shown the means to electrically energize the part 30 consists of a metal electrode 34 at the lower end, and another metal electrode 36 at the upper end. The electrical conductors 38 and 40 are connected to the electrodes. The parts 30 and 32 are separate blocks of ferroceramic material which are secured together, as by means of a cement bond, preferably an epoxy cement, at their interface. The parts 30 and 32 preferably are made of identical material, so that they have the same coefficient of expansion, and the same resonance frequency.

For purposes of illustration, the overall vertical height of the two blocks 30 and 32 may be maintained at two inches, in which case the natural fundamental resonant frequency of the lower block 30 in free space would be twice that of the two inch block shown in FIG. 1, or 80,000 cycles per second (cps). When solidly joined to block 32, however, a composite tranducer is formed wherein the overall length resonance of the structure is reduced to 40,000 c.p.s. The surface with the arrow will oscillate, and forms part of an isolation slug which can be afiixed to a load by more positive means than heretofore found possible. Section 32 is inert and does not vibrate of its own accord. Therefore, it need not be polarized to provide a piezoelectric effect, and thus may be handled as an ordinary ceramic at temperatures well above those allowable to aflix with high temperature cement the surface 14 (FIG. 1) to a load. Non-polarization has the further advantage of avoiding possible electric shock by reverse piezo efiect. Furthermore, and most important, interface 36 in FIG. 2 is not subjected to stress set up by differential expansion, and remains reliable despite wide sudden temperature change. The use of the inert block 32 also provides a considerable measure of heat isolation which is of great value in cases where very hot liquids are in near proximity to the surface to be attached, in this case the top end. Thus a low temperature epoxy cement may be used at interface 36.

A modified form of the invention is shown in FIG. 3. This has the advantage that the lower or transducer portion 42, and the upper or isolation portion 44, are parts of a single integral block of transducer material. Thus, for a frequency of 40,000 c.p.s. the block shown in FIG. 3 may be about two inches high, and may be identical with that shown in FIG. 1, except that the upper electrode is entirely different.

In FIG. 3 the lower electrode is a coating of metal at 46. The upper electrode 48 is a peripheral strip of metal around the body between its ends. In preferred form it has a coating of metal located about halfway between the ends. A conductor 50 is connected to a tab 52 at one end of the electrode 46, while another conductor 54 is connected to the band 48.

It may be mentioned that in manufacture of the transducer assembly the electrodes 46 and 48 are used when polarizing the transducer portion 42. At this time the assembly may be inserted in a holder which provides a third electrode at the end 56 of the isolation portion 44. A short circuiting connection is preferably provided between the third electrode at 56 and the electrode 48 during the polarizing step. This short circuit effectively prevents polarization of the isolation portion 44, with advantages pointed out above.

, Inthis form of the invention, the interface between two similar blocks is completely eliminated. A. single solid block of ferroceramic material has the pair of electrodes 46 and 48. Electrode 46 is silvered in conventional fashion. Electrode 48 is a thin band silvered around the four sides of the block, preferably midway between the ends. The electrodes are applied by means of conventional high temperature silver deposition techniques. Thereafter the block is polarized by the application of a high direct current voltage between the electrodes 46 and 48, which aligns the molecules and makes only the lower half of the block piezoelectric. In order to prevent polarization of the upper half, the top face may be held in contact with a metal plate which is held at the same potential as the center band during the polarization period. Thereafter, in use, the application of an alternating voltage to terminals 46 and 48 will provide performance identical to that achieved with the assembly shown in FIG. 2. However, the bond between blocks 30 and 32 required in FIG. 2 has been entirely eliminated, at a saving in labor, and with a great increase in reliability.

Referring now to FIG. 4, the transducer assembly there shown is like that shown in FIG. 3 except for the provision ofa peripheral ledge or collar 60. This is made of transducer material, barium titanate in the specific case here shown, and is preferably cast integrally with the transducer portion 62 and isolation portion 64 of the assembly. In other words, the transducer 62, isolation slug 64, and collar 60, are all molded or cast integrally as a single body of transducer material. An electrode 66 is provided at one end of transducer 62, and

a peripheral electrode 68 is provided at the other end.

to prevent polarization, all as described above inconnection with FIG. 3.

The main difference is in the mounting of the assembly, as may be explained in connection with FIG.'6. We there show a cleaning apparatus comprising a tank 74 in a supporting housing '76. The bottom of the tank is vibrated by means of a transducer 78, the upper end of which is cemented or otherwise secured to the bottom of the tank. Heretofore the weight of the transducer was supported by the bond to the tank, that is, the transducer was suspended from the tank. The increasing use of barium titanate, and the ever increasing power and size the transducer blocks used, creates a problem in that the bond between the transducer and the tank must withstand both the force of vibration and the added gravitational load of the large transducer blocks.

In the apparatus shown in FIG. 6, however, the weight of the transducer is supported on a support resting on the bottom 82 of the apparatus. The support 80 receives the collar 84, and the dimensioning of the parts is such that the support 80 takes the dead weight load.

The transducer may be and preferably is a half-wave long, with the collar 34 located midway between the ends, and therefore at a nodal point which is stationary. The ends are free for vibration. With a block of different length the collar should nevertheless be located at a nodal point.

With the arrangement shown in FIG. 6 the presence of high temperature liquid in the tank does no harm. The possibility of destroying polarization is of no consequence because the upper half of the transducer block is unpolarized, while the lower half is insulated from the high temperature by the upper half. The high temperature will not open the cement bond to the block because a high temperature epoxy cement may be used, and that in turn is possible because there is no concern over loss of polarization when using the high temperature cement. A similar remark would apply to the use of a soldered connection to a fired metal coating, which also is possible.

FIG. 5 shows a difierent form of transducer assembly which may be used for a very different purpose. In this case the assembly is cylindrical, and maybe quite small,

say a fraction of an inch in diameter and length. It may be used as a sensor element to determine the presence, absence, or level of a liquid material, as disclosed in the copending application Serial No. 656,372 mentioned above. The sensor is normally used with the isolation slug 90 at the bottom, and the transducer portion 92 at the top. The collar 94 may be formed integrally with the parts 90 and 92, all being made, for example, out of barium titanate ceramic. The upper electrode 96 is a coating of fired silver at the end of the transducer portion 92, while the lower electrode 98 is a peripheral band of fired silver located at the lower end of the transducer portion 92. Conductors 100 and 102 lead to the electrodes, and all of these parts may be housed within a cylindrical housing, suggested by the broken lines 104, the said housing preferably being secured to the annular collar 94. The collar is preferably located at a vibration nodal point.

Polarizing voltage is applied across leads 100 and 102 during manufacture, and part 90 is short circuited to prevent polarization, as described above. Alternating voltage is applied during operation, or in a reciprocal case, alternating voltage is developed. The load end is at the bottom. The nodal support allows the two ends to vibrate freely.

In the aforesaid application Serial No. 656,372, the exposed portion 90 beneath the cylindrical housing 104 is a metal slug, and this must be cemented to the transducer material. In the present arrangement, even if a cemented junction were employed, as in FIG. 2, the parts 90 and 92 would be made out of identically the same transducer material, which eliminates the problem of differential expansion or contraction when changing to high or low temperature. Even better, the parts 90, 92 and 94 all may be formed out of a single integral piece of transducer material.

In FIG. 7 the invention is shown applied to a soldering iron, the tip 112 of which is vibrated at ultrasonic frequency by a transducer 110. Such soldering irons are already known, but there is a problem in attempting to secure the transducer 110 to the hot soldering tip 112, the latter being heated by a high resistance heating element 114. In accordance with the present invention an isolation slug 116 is disposed between the soldering tip and the transducer 110, and the isolation means 116, is preferably made of the same transducer material as the part 110. Even better, they preferably are parts of a single body of transducer material with the upper elec trode 118 on one end of the transducer portion 110, and the other electrode 120 formed as a peripheral strip or band of metal, all as previously described.

In FIGS. 2 through 7 the transducer body may be one-half wave long, with the upper and lower portions each one-quarter wave long. This is not essential, however, as the transducer body might, for instance, be a full wave long, in which case the energized and isolation portions would each be one-half wave long. However, in such case, ifa mounting collar be provided, it preferably would not be located at the same point as the electrode. Instead, such a collar is preferably located at a nodal point, and therefore with a composite body which is a full Wave long, the collar is preferably located onequarter wave from the end.

It is not essential that the transducer be piezoelectric, or ferroceramic. It may be of the magnetostrictive type, and such an arrangement is schematically. illustrated in FIG. 8, in which the assembly comprises a transducer portion 122 formed integrally with an isolation portion 124. These are both made of laminations of magnetostrictive material, typically a nickel alloy designed for the purpose. The lower part 122 is energized by a suitable winding, and in the present case there are two coils 126 and 128 which may be connected to a common power source. The resonance frequency of the assembly will be determined by the overall dimension, but only the lower portion nonpolarized.

More specifically, the lower or transducer half of the core is magnetized, and usually this is done by applying a direct current to the coils shown. The upper or isolation portion is preferably not magnetized, and this desirable result is obtained automatically because the fiux path when magnetizing the lower half forms a closed circuit without entering the upper half. The resulting isolation serves much the same useful purpose as previously described, because high temperature tends to destroy the necessary D.C. magnetization of the core. However, the interposed isolation portion is non-energized and nonmagnetized.

The invention may be used with such common piezoelectric materials as quartz, Rochelle salts, ammonium dihydrogen phosphate, and potassium dihydrogen phosphate, in addition to the commonly encountered ferroceramic materials. Magnetostrictive transducers fabricated from nickel or nickel alloy laminations surrounded by coils carrying alternating current may also be made in the composite form shown, thereby eliminating the need for a difficult bonding operation hitherto found necessary.

In all the cases mentioned, the transducer may be used as a generator or receiver of vibratory energy because, as is well known, such devices display reciprocal characteristics.

It is believed that the construction and method of use of our improved transducer assemblies, as well as the advantages thereof, will be apparent from the foregoing detailed description.

Our transducer configuration will withstand wide temperature gradients, and will effectively isolate the vibratory element itself from extremes of temperature. Further, the technique appreciably simplifies the construction of such devices.

It will be apparent that while we have shown and described our invention in several preferred forms, changes may be made in the structures shown without departing from the scope of the invention as sought to be defined in the following claims.

We claim:

1. A combined transducer and isolation slug assembly, said assembly being made of one kind of transducer material dimensioned for vibration at the desired frequency, and comprising both transducer and isolation portions disposed end to end and having the same crosssectional dimension, an electrode at one end of the assembly acting as an electrode at one end of the transducer, a second electrode intermediate the ends of the assembly acting as an electrode for the other end of the transducer, the arrangement being such that the transducer portion is electrically energized while the isolation portion is not energized, a mounting collar disposed peripherally around the assembly intermediate its ends, the transducer portion and the isolation portion and the collar being all made integrally out of a single body of transducer material, said first-named electrode being metal at one .end of said body, said second electrode being a band of metal extending peripherally around the body intermediate its ends.

2. A combined transducer and isolation slug assembly, said assembly being made of one kind of transducer material dimensioned for vibration at the desired frequency, and comprising both transducer and isolation portions disposed end to end and having the same crosssectional dimension, an electrode at one end of the assembly acting as an electrode at one end of the transducer, a second electrode intermediate the ends of the assembly acting as an electrode for the other end of the transducer, the arrangement being such that the transducer portion is electrically energized while the isolation portion is not energized, a mounting collar disposed peripherally around the assembly intermediate its ends, the transducer portion is polarized, while the upper portion is and the isolation portion and the collar being all made integrally out of a single body of transducer material, said first-named electrode being metal at one end of said body, said second electrode being a band of metal extending peripherally around the body intermediate its ends immediately adjacent said collar.

3. A combined transducer and isolation slug assembly, said transducer and said isolation slug portions of said assembly both being made of the same transducer material and both together being dimensioned to be one-half wave long for vibration at the desired frequency, and comprising both transducer and isolation portions disposed end to end and having the same cross-sectional dimension, an electrode at one end of the assembly acting as an electrode at one end of the transducer, a second electrode at the middle of the assembly acting as an electrode for the other end of the transducer, the arrangement being such that the transducer half is electrically energized while the isolation half is not energized, the half between the electrodes being polarized, and the isolation half being unpolarized, a mounting collar disposed References Cited in the file of this patent UNITED STATES PATENTS 1,865,858 Hund July 5, 1932 2,589,403 Kunie Mar. 18, 1952 2,622,470 Rines Dec. 23, 1952 2,697,936 Farrow Dec. 28, 1954 2,839,695 Robey June 17, 1958 

